The objective of the proposed studies is to gain a comprehensive understanding of the lineal relationships, activation requirements, and functional capabilities of subpopulations of human Leu-3+ helper T (Th) cells. These studies will exploit the availability of two novel monoclonal antibodies, HB-10 and HB-11, and are designed to test the hypothesis that the immunoregulatory functions of human Leu-3+ Th cells are performed by distinct subsets which are recognizable by the expression of the HB-10 and HB-11 antigens and which represent sequential stages in post-thymic differentiation. Our specific aims are to determine whether functionally immature Leu-3+HB-10+ cells can be induced to differentiate in vitro into functional helper T cells expressing the Leu-3+HB-10- phenotype. Subpopulations of Leu-3+ cells purified by fluorescence activated cell sorter (FACS) techniques will be stimulated with polyclonal activators or alloantigenic stimuli to determine the activation stimuli, anabolic activity, and accessory cells required for inducing their phenotypic and functional maturation. The kinetics of this differentiation process and the need for cell division will also be determined. Studies will be performed to determine whether the deficiency of the Leu-3+HB-10- T cell subpopulation in patients with X-linked agammaglobulinemia reflects a primary defect in T cell differentiation or a secondary aberration due to the absence of B cells. The immunoregulatory interactions of FACS-purified Leu-3+HB-10+ and Leu-3+HB-10- cells with Leu-2+ cells will be analyzed in B cell differentiation assays to determine the maturational stage of Leu-3+ suppressor-inducer cells and the characteristics of the Leu-3+ cells that interact with Leu-2+ contrasuppressor cells. The molecular weights, subunit structure, and interrelationship of the HB-10- and HB-11-reactive membrane molecules will be assessed by gel electrophoresis and sequential immunoprecipitation experiments. Finally, the phenotypic characteristics of Leu-3+ cells in patients with autoimmune or chronic inflammatory disorders as well as immunodeficiency diseases will be assessed. The definition of the ontogeny and functions of human immunoregulatory T cell subpopulations and the ability to assess post-thymic Th cell maturation in humans should enhance our understanding of the mechanisms by which quantitative or functional abnormalities of helper T cell subpopulations accompany or contribute to immunologial disorders.